Contemporary models are shown to significantly underestimate the attainable efficiency of solar energy conversion to water splitting, and experimentally a cell containing illuminated AlGaAs/Si RuO2/Pt-black is demonstrated to evolve H-2 and O-2 at record solar-driven water electrolysis efficiency. Under illumination, bipolar configured Al0.15Ga0.85As (E-g = 1.6 eV) and Si (E-g = 1.1 eV) semiconductors generate open circuit and maximum power photopotentials of 1.57 and 1.30 V, well suited to the water electrolysis thermodynamic potential: H2O --> H-2 + 1/2O(2); E-H2O(degrees) = E-O2 - E-H2; E-H2O(degrees)(25 degreesC) = 1.229 V. The E-H2O(degrees)/photopotential matched semiconductors are combined with effective water electrolysis O-2 or H-2 electrocatalysts, RuO2 or Pt-black The resultant solar photoelectrolysis cell drives sustained water splitting at 18.3% conversion efficiencies. Alternate dual bandgap systems are calculated to be capable of attaining over 30% solar photoelectrolysis conversion efficiency.